1. Field of the Invention
The present invention relates to an image forming apparatus that controls the speed of a motor (polygon motor), which rotatively drives a rotary polygon mirror.
2. Description of the Related Art
Conventionally, it has been known that, when controlling the rotational speed of a motor (hereinafter referred to as “polygon motor”) that rotates a rotary polygon mirror in an electrophotographic laser beam printer or the like, the rotational speed of the polygon motor is controlled such that the period of a main scanning synchronizing signal (hereinafter referred to as “the BD signal”) generated based on a laser beam reflected from a reference position (for example, the leading end position) of each mirror plane of the rotary polygon mirror is made equal to the target period.
An exposure-scanning system of an electrophotographic printer is constructed as shown in FIG. 1. Referring to FIG. 1, a laser unit 102 turns on/off a laser beam according to an input image signal (video signal) 101 on a dot-by-dot basis. A laser beam 103 emitted from the laser unit 102 is irradiated on a rotary polygon mirror 105. A polygon motor 104 rotatively drives the polygon mirror 105 to deflect the laser beam 103.
An image-formation lens 106 is operable to focus a deflected laser beam 107 on a photosensitive drum 108. The irradiation of the laser beam 107 forms an electrostatic latent image corresponding to the image signal 101 on the photosensitive drum 108. The electrostatic latent image is developed as a toner image by a developing device, not shown, and transferred onto a recording sheet.
The polygon motor 104 rotates the polygon mirror 105 at a high speed of about 10000 to 20000 rpm. The photosensitive drum 108 is rotated one dot in a longitudinal direction (sub-scanning direction) while the laser beam 107 is swung once in the horizontal direction as viewed in FIG. 1 by one mirror plane of the polygon mirror 105, that is, while the photosensitive drum 108 is exposure-scanned one line in the main scanning direction by the laser beam 107.
A reflecting mirror 120 is disposed at such a location that the laser beam 107 is irradiated onto the reflecting mirror 120 when the leading end position in the main scanning direction is subjected to exposure scanning by the laser beam 107. The laser beam 107 reflected from the reflecting mirror 120 falls on a photoelectric conversion element 109. The photoelectric conversion element 109 performs photoelectric conversion of the incident laser beam 107 and outputs the result as the BD signal.
The BD signal is transmitted to a control circuit 111, which provides various kinds of control such as control of the rotation of the polygon motor 104, via a cable 110. Incidentally, as can be presumed from the above description, the BD signal is generated pulse by pulse for respective mirror planes of the polygon mirror 105. According to this prior art, since it is assumed that the polygon mirror 105 has six mirror planes, the BD signal is generated six times per rotation of the polygon mirror 105.
A description will now be given of a method of controlling the speed of the polygon motor 104 according to the prior art. FIG. 2 is a circuit block diagram showing the control circuit 111 in FIG. 1, and the polygon motor 104 and its related parts in FIG. 1 are schematically added to FIG. 2. Note that the reflecting mirror 120 and its related parts for detecting the BD signal are omitted from FIG. 2.
As shown in FIG. 2, the BD signal is inputted to a frequency-dividing circuit 11. The frequency-dividing circuit 11 frequency-divides the BD signal by a value equal to the number of the mirror planes of the polygon mirror 105. According to the prior art, since it is assumed that the polygon mirror 105 has six mirror planes, the BD signal is frequency-divided by 6. The speed of the polygon motor 104 is then controlled based on the period of the BD signal (i.e. BD/6 signal) frequency-divided by the frequency-dividing circuit 11.
A description will now be given of the reason why the BD signal is frequency-divided. The mirror planes of the polygon mirror 105 are not identical with each other but differ in length, profile irregularity, and so forth. Thus, even when the polygon motor 104 is steadily rotating, there is a variation in the period of the actual BD signal.
FIG. 3A shows the state of the actual BD signal. Assuming that the polygon mirror 105 has six mirror planes, the BD signal shows periods T1, T2, T3, T4, T5, and T6 as it is generated by the respective mirror planes as shown in FIG. 3A, and then the respective BD signal pulses of these periods are cyclically generated.
In this case, even when the polygon motor 104 is rotating at a target speed, the speed of the polygon motor 104 cannot be properly controlled based on the BD signal if there is a variation in the periods T1-T6 of the respective BD signal pulses as shown in FIG. 3A.
On the other hand, by frequency-dividing the BD signal by 6 to generate the BD/6 signal, the BD signal can be shaped into one BD signal pulse (BD/6 signal) per rotation of the polygon motor 104 as shown in FIG. 3B. In this case, if the rotational speed of the polygon motor 104 is maintained at the target speed, the BD/6 signal has a constant period (Tround) without being affected by a variation in the profile irregularity of the mirror planes of the polygon mirror 105.
In other words, the BD/6 signal enables the rotational period of the polygon motor 104 to be accurately measured without being effected by a variation in the profile irregularity of the mirror planes of the polygon mirror 105. For the reasons explained above, the BD signal is frequency-divided by a value equal to the number of mirror planes of the polygon mirror 105, and the frequency-divided BD signal is used as a reference signal for detecting the speed of the polygon motor 104.
As shown in FIG. 2, the BD signal (BD/6 signal) divided by the frequency-dividing circuit 11 is inputted to a counter 12. The counter 12 is comprised of an up-counter that counts clocks, not shown, and is configured to clear its count value to measure the period of the BD/6 signal, i.e. the rotational period of the polygon motor 104 each time the BD/6 signal is inputted to the counter 12.
A comparator 13 compares the count value of the counter 12 (BDprd signal) and the target speed (Vtgt) with each other, and generates a control signal for instructing the polygon motor 104 to accelerate or decelerate according to the comparison result. It should be noted that a target speed corresponding to the BD/6 signal is set as the target speed Vtgt.
FIG. 4 shows the functions of an acceleration instruction signal (ACC signal) and a deceleration instruction signal (DEC signal) as output signals from the comparator 13.
Both the ACC signal and the DCC signal are high active signals, and as shown in FIG. 4, if only the ACC signal is at a high level, it means that the polygon motor 104 is instructed to accelerate, and if only the DEC signal is at a high level, it means that the polygon motor 104 is instructed to decelerate. If the ACC signal and the DEC signal are at the same level, it means that the polygon motor 104 is instructed to maintain its speed.
A description will now be given of examples of the operation of the comparator 13 with reference to timing charts of FIGS. 5A-5D and 6A-6E. In the examples shown in FIGS. 5A-5D and 6A-6E, the target speed (Vtgt) is set to 80. FIGS. 5A-5D show a case where the speed of the polygon motor 104 is lower than the target speed, and FIGS. 6A-6E show a case where the speed of the polygon motor 104 is higher than the target speed.
First, a description will be given of the case where the speed of the polygon motor 104 is lower than the target speed. As shown in FIG. 5A, in response to the input of a falling edge of the BD/6 signal, the counter 12 clears its count value (BDprd) to 0 (FIG. 5C). The counter 12 then increments its count value to 1, 2, 3, . . . in synchronism with clocks, not shown, and if the count value becomes greater than the value of 80 set as the target speed Vtgt, the comparator 13 outputs the ACC signal at a high level until the next falling edge of the BD/6 signal is inputted (FIG. 5D).
In this case, the lower the speed of the polygon motor 104 relative to the target speed, the longer the period of the BD/6 signal. It follows that the lower the speed of the polygon motor 104, the longer the high level width of the ACC signal.
A description will now be given of the case where the speed of the polygon motor 104 is higher than the target speed. As shown in FIG. 6A, assuming that the count value is 77 when a falling edge of the second BD/6 signal is inputted (FIG. 6C), the comparator 13 outputs the DEC signal with a high level width corresponding to an amount by which the speed of the polygon motor 104 is lower than the target speed of 80 (FIG. 6E).
In this case, the higher the speed of the polygon motor 104, the shorter the period of the BD/6 signal. It follows that the higher the speed of the polygon motor 104 relative to the target speed, the longer the high level width of the ACC signal.
If the speed of the polygon motor 104 is equal to the target speed, that is, if the period of the BD/6 is equal to Vtgt, both the ACC signal and the DEC signal are maintained at a low level.
As shown in FIG. 2, the ACC signal and the DEC signal generated by the comparator 13 are inputted to the motor driving section 14. The motor driving section 14 is comprised of constant current sources 19 and 20, switching elements 16 and 17, a charge pump capacitor 15, and an amplifier 18.
The constant current sources 19 and 20 and the switching elements 16 and 17 constitute a charge-discharge circuit for the charge pump capacitor 15. When the DEC signal goes high, the switching element 16 is turned on or closed to charge the charge pump capacitor 15 via the constant current source 19. When the ACC signal goes high, the switching element 17 is turned on or closed to discharge the charge pump capacitor 15 via the constant current source 20.
Therefore, the voltage of the charge pump capacitor 15 is increased or decreased in proportion to the high level width of the ACC signal and the DEC signal. The resulting voltage is transmitted to the motor driver 21 via the amplifier 18. The motor driver 21 supplies a current proportional to the voltage to the polygon motor 104, thus rotating the polygon motor 104.
If the rotational speed of the polygon motor 104 is lower than the target speed, the voltage of the charge pump capacitor 15 is increased to accelerate the polygon motor 104 because the ACC signal is at a high level. Conversely, if the rotational speed of the polygon motor 104 is higher than the target speed, the voltage of the charge pump capacitor 15 is decreased to decelerate the polygon motor 104 because the DEC signal is at a high level.
If both the ACC signal and the DEC signal are at a low level, the voltage of the charge pump capacitor 15 remains unchanged and the rotational speed of the polygon motor 104 is maintained since the switching elements 16 and 17 are off. As a result, the rotational speed of the polygon motor 104 stays at the target speed.
The conventional polygon motor control circuit 111, however, has the possibility that the rotational speed of the polygon motor 104 greatly varies because a variation in the rotational speed of the polygon motor 104 is detected at long time intervals corresponding to six pulses of the BD signal.
Further, due to long time intervals at which the ACC signal and the DEC signal are generated, it is difficult to tune the motor driving section 14 for steadily rotating the polygon motor 104. Specifically, the conventional polygon motor control circuit 111 has the problems, for example, that it is difficult to secure the voltage stability of the charge pump capacitor 15 and to adjust the acceleration/deceleration in response to one accelerating/decelerating instruction.